Function diameter_with_mode 

pub fn diameter_with_mode(
    graph: &Graph,
    mode: EccMode,
) -> IgraphResult<Option<u32>>

Diameter of graph under the given mode. None for vcount == 0.

Counterpart of igraph_diameter(_, NULL_weights, _, NULL, NULL, NULL, NULL, mode == directed ? IGRAPH_OUT : IGRAPH_ALL, /*unconn=*/true).